Hammer drills are power tools that generally have three modes of operation, i.e. a hammer only mode, a drill only mode and a combined hammer and drilling mode. In general, the motor of a hammer drill is operated by the user depressing a spring-loaded trigger, and deactivated by the user releasing the trigger such that it is necessary to hold the trigger down during operation of the tool.
U.S. Pat. No. 6,109,364 describes a rotary hammer drill which has three modes of operation, namely a purely drilling mode, a purely hammering mode and a combination of drilling and hammering mode. A mechanism is provided by which the rotary hammer can be switched between the three modes of operation.
It is desirable for such tools to be able to be “locked on” in the pure hammering mode only. This means that when the pure hammer mode is selected and the trigger button is depressed, the hammer can be “locked on” so that the removal of the fingers from the trigger button does not cause the tool to switch off but it in fact continues operating within the pure hammer mode until the “lock on” mechanism is deactivated. However, it is undesirable that such a feature is capable of being activated when in either the rotary only mode of operation or in the combination of the rotary and hammering mode of operation. Therefore, rotary hammers are constructed so that they can only be “locked on” when in the pure hammer mode only. GB2314288 describes one such mechanism whereby the trigger button is mechanically locked on in the hammer only mode.
The present invention provides an alternative design to the “lock on” mechanism in GB2314288.